1. Field of the Invention
The present invention relates to a semiconductor memory device, for example, a NAND-type flash memory.
2. Description of the Related Art
A NAND-type flash memory (for example, Jpn. Pat. Appln. KOKAI Publication No. 2002-324400, Jpn. Pat. Appln. KOKAI Publication No. 8-64699, and Jpn. Pat. Appln. KOKAI Publication No. 8-115987) has been known as one of nonvolatile semiconductor memory devices.
In the NAND-type flash memory, a plurality of serially connected EEPROM cells comprise, for instance, a NAND unit. A source side of the NAND unit is connected to a source line through a first selection gate, and a drain side is connected to a bit line through a second selection gate. A diffusion layer of the second selection gate and the bit line are connected to each other through a bit line contact.
The bit line contact is formed as below. A first and a second selection gates and word lines are formed firstly. After this, control gate electrodes as word lines are silicided. Insulating films are deposited between word lines. Bit line contacts then formed through a generic contact forming technique, such as a lithography process, an etching process, a depositing process of metallic films, and a planarizing process.
Electrode member as the bit line contacts are embedded in bit line contact holes. However, recently, as fineness in an element is promoted, mutual intervals between adjacent bit lines become narrower. Therefore, it has become hard to form fine contact holes and to embed the electrode members in the contact holes, and the connections between the bit line contacts and the diffusion layers of the second selection gates become defective. Therefore, poor connections possibly occur.
Enlarging the diameters of the contact holes is a possible approach to prevent the poor connections of the bit line contacts. However, in this case, the adjacent bit line contacts make contact with each other, and results in generation of short circuit of the NAND units.
Accordingly, a semiconductor memory device which facilitates the forming of the bit line contacts, and allows writing therein with sure has been desired.